Removal of sodium chloride from pulp mill operations

ABSTRACT

Sodium chloride is removed from pulp mill systems in which the smelt from the spent pulping liquor recovery operations contains sodium carbonate and sodium chloride, and optionally other components, depending on the pulp mill system. The smelt is made up into a hot aqueous solution thereof, and is evaporatively cooled to crystallize hydrated sodium carbonate, while avoiding deposition of sodium chloride. After separation of the crystallized sodium carbonate, and other crystallized salts, for example, sodium sulfide and sodium sulfate if a Kraft mill system is employed, the mother liquor is evaporated with heating to deposit sodium chloride in a substantially pure form. The sodium carbonate may be converted into an active pulping chemical for recycle, typically into sodium hydroxide by recausticization.

United States Patent Lukes [451 Sept. 30, 1975 [75] Inventor: Jerome A.Lukes, Ogden, Utah [73] Assignee: Erco Envirotech Ltd., Canada [22]Filed: Dec. 3, 1973 [21] Appl. No.: 420,876

[52] US. Cl. 162/19; 162/30; 162/89; 423/207; 423/421; 423/499; 162/17[51] Int. Cl. D21C 3/02 [58] Field of Search 162/17, 30, 36, 29, 88,162/89, DIG. 8, 19; 423/207, 183, 184, 499, 421

[56] References Cited UNITED STATES PATENTS 1,791,281 2/1931 Morse423/184 X 2,642,399 6/1953 Aries et al. 423/207 X 2,862,887 12/1958Boyer 162/30 X 3,396,076 8/1968 Crosby et a1 162/33 3,560,329 2/l97 lNelson et al 162/30 3,698,995 10/1972 Rapson l62/DlG. 8 3,746,612 7/1973Rapson et al. 162/17 3,755,068 8/1973 Rapson 162/30 FOREIGN PATENTS ORAPPLICATIONS 928,008 6/1973 Canada l62/D1G. 8

WOOD

PULP

BLEAC H PLANT Primary Examiner-S. Leon Bashore Assistant Examiner-PeterF. Kratz Attorney, Agent, or F irmShoemaker and Mattare, Ltd.

[57] ABSTRACT Sodium chloride is removed from pulp mill systems in whichthe smelt from the spent pulping liquor recovery operations containssodium carbonate and sodium chloride, and optionally other components,depending on the pulp mill system. The smelt is made up into a hotaqueous solution thereof, and is evaporatively cooled to crystallizehydrated sodium carbonate, while avoiding deposition of sodium chloride.After separation of the crystallized sodium carbonate, and othercrystallized salts, for example, sodium sulfide and sodium sulfate if aKraft mill system is employed, the mother liquor is evaporated withheating to deposit sodium chloride in a substantially pure form. Thesodium carbonate may be converted into an active pulping chemical forrecycle, typically into sodium hydroxide by recausticization.

33 Claims, 2 Drawing Figures PULP g/CLO BROWN '5 STOCK WASHER NaCL Na COSOLUTIONfi 44 0 2 5 38 V 48 1 H2O FURNACE SMELT SMELT DISSOLVER 42 Z 2CAUSTICIZER DISSOLVER cal) cLco 564 3 66 LIME KlLN PULP 24 22\ BLEACH 22 18 PLANT H O Na OH WOOD CHIPS UNBLEACHED BPE PULP 1o 1A "20 34 4O H OBROWN SPENT DIGESTER STOCK EVAPORATOR PULPING 14 WASHER LIQUOR PULPINGLIQUOR NaCL Na CO SOLUTION 1 H20 48 A 38 T SMEI-T sMEl-T HOT AQUEOUSFURNACE DISSOLVER SOLUTION CRYSTALLIZER H20 52 O t A EYAPORATOR Na CO l6 58 8 Z 1 6O 2 76 NaCL NaOH SOLUTION CAUSTICIZER 4L DISSOLVER I 56% CaOCaCO H2O 66 US. Patent Sept. 30,1975 Sheet 2 012 5 LT 112 ME SMELT H2ODISSOLVER HOT GREEN LIQUOR 118 CARBONATOR L'CO2 PRECOOLER l NaCl.PRECOOLED SOLUTION EVAPORATOR #1120 1 12 EVAPORATIVE COOLER THICKENERBURKEITE A SLURRY RE Y E 132 CCL MOTHER SEPARATOR LIQUOR Na CO Nal2s 136 N82 S04 FILTRATE EVAPORATOR TO RECAUSTICIZATION FIG. 2

REMOVAL OF SODIUM CHLORIDE FROM PULP 'MILL OPERATIONS FIELD OF THEINVENTION The present invention is directed to the removal of sodiumchloride, from pulping operations, more particularly from chemical woodpulping operations including bleaching procedures for the pulp, withenhanced efficiency of pulp mill operations.

BACKGROUND TO THE INVENTION Generally in the production of pulp suitablefor formation into paper, wood or other raw cellulosic fibrousmaterials, is subjected to chemical digestion in a pulping liquor toform a pulp of the cellulosic fibrous material. The pulp thereafter issubjected to brightening and purification operations in a bleach plant.

The spent pulping liquor usually is subjected to a series of recoveryand regeneration operations to recover pulping chemicals and to providefresh pulping liquor. Generally, the pulping liquor contains alkalimetal salts, usually sodium salts, although potassium or lithium saltscould be employed.

Two well-known pulping processes are the Kraft and soda processes. Whilethe present invention will be described hereinafter with particularreference to Kraft and soda mills, the process is applicable also toother pulp mill operations involving the use of alkali metal salts aspulping chemicals and which utilize spent chemical recovery and pulpingchemical regeneration operations. Many such operations involve the useof sulphur-containing chemicals, including the Kraft process, high yieldpretreatment Kraft process, alkaline sulfite process, neutral sulfiteprocess, acid sulfite process, bisulfite process, acid bisulfiteprocess, polysulfide process, alkafide process. Other pulping operationswhich may be employed are the soda-oxygen process and the holopulpingprocess. The present invention also is applicable to cross-recoveryoperations used in mixed systems.

In the conventional Kraft process, raw cellulosic fibrous material,generally wood chips, is digested, by heating, in a pulping liquor,known as white liquor and containing sodium sulphide and sodiumhydroxide as the active pulping chemicals, to provide a pulp and spentpulping liquor, known as black liquor. The black liquor is separatedfrom the pulp by washing in a brown stock washer and the pulp then ispassed to the bleach plant for brightening and purification operations.

The black liquor then is passed to the recovery and regeneration systemin which the black liquor first is concentrated, usually by evaporation,and the concentrated black liquor is burned in a furnace to yield asmelt containing primarily sodium carbonate and sodium sulphide. Asodium-and sulphur-containing compound, generally sodium sulphate, isadded to the black liquor generally prior to feed of the concentratedblack liquor to the furnace, although such sodium-and sulphur-containingcompound may be added at any other convenient point, such as to thewhite liquor prior to the digestion step, to make up sodium and sulphurvalues lost from the recovery system.

The smelt is dissolved in water to yield a raw green liquor which thenis clarified to remove undissolved solids. The clarified green liquor iscausticized with slaked lime whereby the sodium carbonate is convertedto sodium hydroxide. The resulting liquor is white Iiquor which then maybe recycled to the digestion step to provide at least part of thepulping liquor.

As mentioned above another pulping process to which the presentinvention may be applied is the soda process. In this process, thepulping liquor consists predominantly of aqueous sodium hydroxidesolution. Spent pulping liquor is subjected to a recovery andregeneration procedure as in the Kraft process. The smelt which resultsfrom furnacing in the soda process contains primarily sodium carbonatewhich, after formation into an aqueous solution, is recausticized toregenerate sodium hydroxide solution for recycle to provide at leastpart of the pulping liquor.

Bleach plant operations generally involve a sequence of brightening andpurification steps, together with washing steps. The brightening stepsgenerally involve the use of bleaching agents. At least one of thebrightening steps usually involves theuse of at least onechlorine-containing bleaching agent. Such chlorinecontaining bleachingagents include chlorine, chlorine dioxide, chlorine monoxide and sodiumhypochlorite.

The purification steps generally involve treatment with sodium hydroxidesolution, and usually is known as a caustic extraction step. In someinstances, the bleaching and caustic extraction steps may be combined,for example, using the so-called oxygen bleaching operation.

A particular bleaching operation which has been employed involves aninitial bleaching of the pulp with an aqueous solution containingchlorine or a mixture of chlorine dioxide and chlorine,'- anintermediate washing, a caustic extraction using aqueous sodiumhydroxide solution, a further washing, a bleaching with an aqueoussolution of chlorine dioxide, another washing, a further causticextraction using aqueous sodium hydroxide, an additional washing, afinal bleaching with chlorine dioxide solution and a final washing. Thisis the so-called CEDED operation. The present invention will bedescribed with particular reference to this procedure, although otherprocedures may be employed, such as, the use of an aqueous solutioncontaining approximately percent chlorine dioxide in the first bleachingstep. 4

The above-described CEDED operation may be carried out using theso-called Dynamic Bleaching" process outlined in Canadian Pat. No.783,483. In this process, pulping treating solutions are passedsuccessively through a mat of fibers in which the fibers are maintainedrelatively stationary with respect to each other. Washing steps, exceptfor a washing after the last step of the bleaching and purificationsequence, may be omitted.

The spent wash waters from bleach plants generally have been dischargedto water: bodies, such as streams, rivers, lakes and oceans, without anyattempt to recover chemicals therefrom, although in some instances solidparticle recovery operations have been made. One of the main reasonsthat no attempt has been made to recover these chemicals is because theyare very dilute and of small value. The bleach plant also produces spentbleaching liquor effluents and spent caustic extraction effluents. Theseeffluents have objectionable colour and are toxic and harmful to aquaticand marine biota and polluting since they contain fibers and materialsconsuming oxygen present in the water. It is desirable to avoid suchenvironmental pollution, and hence avoid the discharge of theseeffluents from the mill.

Due to the use of chlorine-containing bleaching chemicals andsodium-containing purification agents, the spent wash water containssubstantial quantities of sodium chloride. In addition, when the spentbleaching liquor effluents and the spent caustic extraction liquoreffluents are mixed at least part of the residual chlorine and sodavalues combine to form sodium chloridev The normally dischargedeffluents, namely. the spent wash water, the spent bleaching chemicalsand the spent caustic extraction liquor may be mixed to provide a bleachplant effluent stream, known as BPE.

The quantity of the chlorine containing bleaching agents and thequantity of sodium hydroxide used as caustic extraction liquorpreferably are balanced to provide about one atom of sodium for eachatom of chlorine, whereby these chemicals form sodium chloride. Inpractice, the quantity of sodium hydroxide solution is in a slightstoichiometric excess to ensure the conversion of all the chlorinevalues to sodium chloride. The equivalence of sodium and chlorine atomsin the bleach plant effluent also is preferred so that the overallsodium inventory of the mill thereby remains unchanged. In the CEDEDsequence when chlorine or mixtures of chlorine dioxide and chlorine inwhich the proportions of available chlorine provided by chlorine dioxideis low, is used in the first stage an amount of sodium hydroxidesolution in excess of that required for extraction must be added tomatch the chlorine atoms present. If no excess is added, only about 40to 50 percent of the first chlorination stage filtrate can be recoveredto match the stoichiometric equivalent of sodium atoms used in theextraction. Where, however, the available chlorine is providedpredominantly by chlorine dioxide, typically above about 70 percent, thequantities of sodium and chlorine atoms are substantially equivalent andhence it is preferred to employ the latter sequence.

Sodium chloride present in the bleach plant effluent also may arise fromsodium chloride present in the pulp when it is introduced to the bleachplant. Such sodium chloride may be present where the logs are floated insea water prior to formation of wood chips therefrom. In addition, ifbrackish water is used to provide the bleach plant wash water, sodiumchloride again is present in the bleach plant effluent.

Sodium salts may be introduced to the pulp mill system, from othersources, such as from the cellulosic fibrous material itself.

In the present invention, the bleach plant effluent preferably is addedto the spent pulping liquor recovery and regeneration operation, and inthis way this effluent is retained within the mill. It has been proposedpreviously in Canadian Pat. No. 832,347 and U.S. Pat. No. 3,698,995 toreduce the environmental problems of bleach plant effluent by utilizingthe spent wash waters to wash the pulp in the brown stock washer. Theuse of the spent wash water in this manner reduces the overall waterrequirement of the mill. In the present invention, it is preferred touse the bleach plant effluent, consisting of a mixture of spent washwaters, preferably provided by countercurrent washing as described inCanadian Pat. No. 832,347 and U.S. Pat. No. 3,698,995. spent bleachingchemicals and spent caustic extraction liquor, to wash the pulp in thebrown stock washer and provide thereby an effluent-free pulp mill and alowering of water requirement.

In a particular manner of carrying out such countercurrent washing in aCEDED sequence in which the pulp is washed after every step, there is acompletely countercurrent flow of liquors, namely spent bleachingliquors, spent caustic extraction liquors and wash water, with respectto the flow of pulp through the bleach plant. In such an operation,fresh water is passed into contact with the pulp following the lastbleaching stage, the spent wash water from this washing being mixed withspent chlorine dioxide from the last bleaching stage. The resultingmixture is split into two streams, the bulk being used to wash the pulpfrom the last caustic extraction stage, and the remainder being used tomix with spent caustic extraction effluent from the last causticextraction stage, the mixture being used partially to mix with effluentfrom the washing step after the intermediate bleaching stage.

The remainder of the mixture is used as wash water to wash the pulp fromthe intermediate bleaching stage. Spent bleaching chemical from thisstage is mixed with the aqueous material resulting from the last mixing,the resulting material, representing the combined effluents from thesubsequent steps of the bleach plant, being used partially to wash thepulp from the first caustic extraction stage and partially to wash thepulp from the first bleaching stage. The spent wash water from thewashing of the pulp from the first caustic extraction stage is mixedwith spent caustic extraction liquor from the first caustic extractionand the mixture is divided into two streams, one of which represents analkaline effluent and the other is used as wash water for the pulp fromthe first bleaching stage. The spent wash water from this stage is mixedwith spent bleaching chemical from the first bleaching stage to providean acid liquor, part of which constitutes an acid bleach plant effluent.

Another part of the acid liquor may be used to mix with the pulpreceived from the brown stock washer to provide the water required tobring the pulp to the consistency necessary in the first bleachingstage. A further part of the acid liquor is used as the aqueous mediumfor dissolving gaseous chlorine used in the first bleaching stage. Inthis way a portion of the acid liquor, constituted by the latter twoparts may be recycled in the first bleaching step.

A further part of the acid liquor may be used as wash water to displaceinto but not through the unbleached pulp on the unbleached decker chestso that it is substantially all discharged to the unbleached deckerchest along with the unbleached pulp.

Still another part of the acid liquor may be used as wash water to washthe screen on the first bleaching stage washer so that it mixes withspent bleaching chemical from the first bleaching stage and is recycledto become part of the acid liquor.

The acid spent bleaching chemical liquor is mixed with the alkalinecaustic extraction liquor to provide the bleach plant effluent.

The bleach plant effluent may be introduced at other stages of therecovery and regeneration operations. Further, the bleach plant effluentmay be split into two or more streams which are introduced at differentlocations of the recovery and regeneration operations, for example, towash calcium carbonate mud to provide the weak wash" water or to diluteconcentrated white liquor.

The quantity of sodium chloride present in the bleach plant effluentvaries depending on the bleaching se- 'quence which is' employed.- Inatypical procedure where a mixture of Chlorine dioxide and chlorine areutilized in the first Stage of a CEDEDsequence the quantity of Sodiumchloride may vary-between about 120 and sgjlbs/t n of'pulp depending onthe propor- "tion of chlorine dioxide used. Typically, when the totalavailable chlorine in the first stage is provided 70 percent by chlorinedioxide and 30 percent by chlorine, t he quantity is about 120 lbs/tonof pulp. Th'e introduction of the bleach plant effluent to the pulpingliquor recovery and regeneration operations closes the whole, system andsodium chloride is'not purged by way of dis carde'd bleach planteffluent. The sodium chloride remains unconverted by the black liquorrecovery steps an d hence'would build up in the system. In order to,prevent such build up and at the same time utilize the concepts of aneffluent-free pulp mill, it is essential to remove sodium chloride fromthe system. Such removal of sodium chloride should be such that theother, valuablecomponents utilizable as or convertible into pulpingfchemicals are not removed from and thereby" lost to the system alongwith the sodium chloride. In addition, it is'prefe rred'to remove aquantity of sodiumichloride from the mill equivalent to "the amountin'troduced 'to and/or produced within the n, typically'about 120lbs/ton pulp..

In instances where bleach plant effluent is not introduced to therecovery and regeneration operations, but is discharged in the normalway, sodium chloride rnay r stilldbe; present in the mill recovery andregeneration I cycle which is required to be removed to prevent buildup. Such sodium chloride may arise from sea-borne ,logs which arepulpedand washed, therebyremoving some sodium chloride, from the pulpand introducing it to the black liquor, prior to passage to the bleachplant. Further, where wash water for washing the pulp prior to passageto the bleach plant is contaminated with sodiurn chloride, at least partof this sodium chloride is introduced to the black liquor. i

The present invention-is concerned broadly, there- I fore, with the,removal. of sodium chloride from pulp mill recovery andlregenerationsystems irrespectiveof the source of such sodium, chloride, and ispreferably ;concerned ,with the removal of sodium chloride fromeffluent-free pulpmills, e

- fl'he process describedand claimed in Canadian Pat. No. 9l5,361 andU.S. Pat. No. 3,746,,6l2 rnakes it feasibletoremove from the millon acoutinuous basis an .--,a mount of sodium chloride equivalent to theamount of chloride introduced to the recovery and regeneration system.This is achieved,by concentrating, preferably by evaporation ,-the whiteliquor prior to recycle to the digestion stage in order to precipitateand removesodium. chloride from-thewhite liquor. 5 The white liquorevaporation process is; outlined in the above-mentioned Canadian Pat.No. 915361 and US; Pat. No. 3,746,612, whileeffectivein. removing sodiumchloride from the system in the desired quantities, neverthelessinvolves considerable energy to evaporate the require'diamount of waterto achieve precipitationof the requisite quantity of sodium chloride and.further involvesconsiderable capital investments in :{special equipmentin view of the high temperature and corrosive nature of the solutions.Further, in the pro- ,-cess of Canadian- Pat. No."9l'5,36l and U.S.Pat..No. 3,746,6l2, '-some sodium chloride remains in the recycled whiteliquor and cycles through the system as a 'dead load.

SUMMARY AND GENERAL DESCRIPTION OF INVENTION lnaccordance with thepresent invention, an alternative sodium chloride separation procedureis provided which, in its preferred embodiments, enables sodium chlorideto be removed from the system at a lower cost.

In accordance with the present invention, a smelt containing sodiumcarbonate and sodium chloride is dissolved in an aqueous material toform hot aqueous solution thereof, thehot aqueous solution is cooled tocrystallize hydrated sodium carbonate therefrom while avoidingprecipitation of sodium chloride, the crystallized hydrated sodiumcarbonate is recovered from the mother liquor,'the mother liquor isevaporated with heating to crystallize sodium chloride therefrom whileavoiding substantially the crystallization of sodium carbonate, and thecrystallized sodium chloride is removed from the resulting motherliquor.

The hot aqueous solution which is cooled bythe procedure of theinvention therefore contains dissolved quantities of the material of thesmelt, including sodium .carbonate and sodium chloride. In someinstances, the smelt may be substantially sodium chloride-free but thehot aqueous solution nevertheless containsdissolved quantities of sodiumchloride as a result of additions of a sodium chloride-containingaqueoussolution, such as bleach plant effluent, to the aqueous solutionof the smelt. Such procedures are within the scope of this invention. I

Further, .it may be desired, under certain circumstances to form a hotaqueous solution for treatment in accordance with the present inventionfrom only part of a sodium chloride-containing smelt and such proceduresare within the scope of this invention.

The hydrated sodium carbonate which is crystallized and recovered inthis way then may be recausticized, after making up into an aqueoussolution thereof, or otherwise reconverted into pulping chemicals. Sincethe sodium carbonate crystallization substantially avoids thecrystallization of sodium chloride, little or no sodium chloride isrecycled with the pulping liquor as a dead load.

The mother li'quor remaining after removal of sodium chloride usuallystill contains dissolved quantities of sodium chloride and sodiumcarbonate, and this mother liquor may be recycled to the aqueoussolution formed from the materials of the smelt.

The quantity of sodium chloride recovered by the process of theinvention preferably is substantially the same quantity as is originallypresent in the smelt, and this may be achieved by judicious choice ofconditions.

Other materials may be present in the smelt, depending on the pulp millsystem which is used. For example, where the Kraft process is used, thesmelt also may contain sodium sulfite and small quantities of varioussodium and sulfur compounds including sodium sulfate, sodium sulfite,sodium thiosulfate and sodium polysulfide. Sodium sulfate is present inthe most significant amount. In one embodiment of the invention asapplied to the Kraft process, at least the sodium sulfide and sodiumsulfate are crystallized along with the sodium carbonate.

The cooling of the hot aqueous solution may be carried out in anyconvenient manner to a temperature which will result in crystallizationof hydrated sodium carbonate, optionally together with any othercrystallizable salts, with the exception of sodium chloride. The form ofthe sodium carbonate which is crystallized depends on a number offactors, including the composition of the cooled solution and thetemperature to which the hot aqueous solution is cooled to causecrystallization. The sodium carbonate is crystallized in a hydratedform, which may be the septahydrate or decahydrate. Preferably thedecahydrate is crystallized. The decahydrate form is crystallized attemperatures below about 25C. Crystallization of the sodium carbonate ina hydrated form in the invention removes water from the system, therebydecreasing the quantity of water which must subsequently be evaporatedto remove water to deposit sodium chloride.

During the cooling step, water preferably is evaporated from the aqueoussolution to increase the concentration of the dissolved salts and thisevaporation may constitute the sole form of cooling the hot aqueoussolution. The cooling effect of the evaporation, carried out undervacuum, may be enhanced in any desired manner.

The smelt may be fractionated during its formation into the hot aqueoussolution to dissolve substantially all the sodium chloride therefromalong with part of the sodium carbonate and part or all of other saltspresent in the smelt, depending on the conditions of the fractionation.The undissolved portion of the smelt is substantially free from sodiumchloride and may be forwarded directly to the recausticization stage.Fractionation of the smelt in this way decreases the energy required todeposit the sodium carbonate from the hot aqueous solution.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a schematic flow sheetillustrating one embodiment of the invention, as applied to a soda pulpmill; and

FIG. 2 is a schematic flow sheet illustrating a further embodiment ofthe invention, as applied to a Kraft pulp mill.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring first to FIG. 1, woodchips, or other raw cellulosic fibrous material, are fed by line to adigester 12 wherein the wood chips are digested with a pulping liquorfed by line 14 and containing sodium hydroxide as the active pulpingchemicalin the soda process.

The resulting pulp and spent pulping liquor are separated and the pulpis washed in a brown stock washer 16. The pulp is washed, in theembodiment illustrated, with aqueous bleach plant effluent fed by line18. Alternatively, the pulp may be washed with water or contaminatedcondensate", and the bleach plant effluent may be utilized elsewhere inthe system, as described in more detail below, or discharged as desired.

The washed and unbleached pulp is fed by line to a bleach plant 22wherein the pulp is subjected to a series of bleaching and purificationprocesses involving the use of one or more chlorine-containing bleachingagents. Generally, the bleaching and purification processes involvebleaching with chlorine, chlorine dioxide or mixtures thereof fed byline 24 and purification by caustic extraction, using aqueous sodiumhydroxide solution fed by line 26, typically in a CEDED sequence asdescribed above. The pulp is washed during the bleach plant operations,typically after each bleaching or caustic extraction operation, by waterfed by line 28. The spent wash water from the bleach plant washingoperations together with the spent chemicals from the bleaching andcaustic extraction steps provide the bleach plant effluent in line 18.

Preferably, the washing operations involve countercurrent flow of pulpand wash water through the bleach plant. Preferably, the quantity ofsodium hydroxide used in the caustic extraction of the pulp is slightlyin excess of the stoichiometric requirement of one sodium atom for eachchlorine atom in the bleaching chemicals, whereby the bleach planteffluent in line 18 has an alkaline pH. The bleached and purified pulpof required brightness is recovered from the bleach plant 22 by line 30and is sold as such, or passed to paper making procedures.

If desired, the bleach plant effluent in line 18 may be added directlyto the spent pulping liquor in line 32, although this procedure is lesspreferred, since the overall water requirement thereby is increased.

The bleach plant effluent in line 18 contains considerable quantities ofsodium chloride which are transferred to the spent pulping liquor inline 32. The spent pulping liquor is evaporated in an evaporator 34prior to passage by line 36 to a furnace 38 of any convenientconstruction. The water recovered from the evaporator 34 by line 40 maybe used to provide at least part of the water requirement of the system,for example, as at least part of the water fed to the bleach plant inline 28, after suitable cleaning, if required.

The spent pulping liquor forms in the furnace 38 a smelt containingsodium carbonate, and additionally unreacted components, consistingsubstantially of sodium chloride.

Thus, there is obtained from the furnacing operation a smelt containingsodium carbonaate and sodium chloride. In accordance with thisembodiment of the present invention, the smelt is dissolved in a smeltdissolver 42 in water fed by line 44. The water may be constituted byBPE, evaporator condensate or weak wash water, if available afterdissolving hydrated sodium carbonate in 54. The water passed by line 44to the smelt dissolver 42 may be partially bleach plant effluent fromline 18, especially in the case where fresh water or contaminatedcondensate is fed to the brown stock washer 16.

The resulting hot aqueous solution in line 46 also contains recycledchemicals as described in more detail below.

If desired, all the smelt need not be dissolved in the aqueous materialfed by line 44, but instead the smelt may be leached with the aqueousmaterial fed by line 44 to dissolve substantially all the sodiumchloride values and part of the sodium carbonate values of the smelt,leaving substantially pure sodium carbonate. Operation in this mannerreduces the quantity of water which requires evaporation. Thesubstantially pure sodium carbonate may be used to form pulping liquoralong with the sodium carbonate later separated from the hot aqueoussolution resulting from the leaching.

The hot aqueous solution is cooled in a crystallizer 48 to crystallizehydrated sodium carbonate, preferably sodium carbonate decahydrate,while avoiding substantially the crystallization of sodium chloride. Thecooling preferably is achieved by evaporation of water from the hotsolution under vacuum, the evaporated water being removed by line 50.

The hot aqueous solution in line 46, which generally has a very hightemperature, typically about 180 to 210F, may be subjected to an initialcooling, with or' without evaporation of water, to reduce the coolingload to be borne by the sodium carbonate crystallizer, prior to furtherevaporative cooling to cause crystallization of the sodium carbonate.More than one such initial cooling steps may be used, if desired. Theinitial cooling may be omitted, if desired.

In a typical operation, the hot aqueous solution at an initialtemperature of about 200F is cooled to about 120F with some evaporationof water and then the solution is evaporatively cooled to below about70F, preferably to about 60F, causing crystallization of sodiumcarbonate decahydrate. Since the sodium carbonate is crystallized in theform of the decahydrate, thereby more water is removed from thesolution.

The evaporative cooling and crystallization of sodium carbonatepreferably are continued until the mother liquor is substantiallysaturated with respect to sodium chloride. The resulting slurrypreferably is diluted with recycled mother liquor to improve theworkability thereof and sodium carbonate, substantially free from sodiumchloride, is separated from the mother liquor by line 52. While thecrystallization of sodium carbonate and its separation from the motherliquor are illustrated as occurring in the same vessel in FIG. 1, thisis for ease of illustration and normally separate vessels are used.

The sodium carbonate is passed by line 52 to a dissolving tank 54wherein it is dissolved in water, or any other suitable aqueous medium,preferably weak wash water, fed by line 56 to form an aqueous sodiumcarbonate solution which is passed to a causticizer 58 by line 60. Thesodium carbonate in the aqueous solution is converted substantiallycompletely to sodium hydroxide by the action of lime fed by line 62 froma lime kiln 64. The calcium carbonate mud precipitated in thecausticizer 58, after suitable washing to remove entrained alkalivalues, is returned to the lime kiln 64 by line 66 for reconversion tolime. The weak wash water resulting from the calcium carbonate mudwashing may, in part be used as the aqueous material fed by line 44 todissolve the smelt.

The aqueous sodium hydroxide solution resulting from therecausticization process is recycled by line 68 to provide at least partof the pulping liquor fed by line 14 to the digester 12.

The mother liquor resulting from separation of the sodium carbonate,containing dissolved quantities of sodium chloride and sodium carbonate,and saturated at least with respect to sodium carbonate, is forwarded byline 70 to an evaporator 72 wherein the mother liquor is heated andwater is evaporated therefrom to deposit sodium chloride, while avoidingsubstantially the crystallization of sodium carbonate. Generally, theevaporation of water in the evaporator 72 is achieved by boiling thesolution under a reduced pressure, preferably until the solution issubstantially saturated with respect to sodium carbonate. The waterevaporated in the evaporator 72 is removed by line 74. i

. The temperature to which the mother liquor is heated to achieve thedesired evaporation and crystallization of sodium chloride depends onthe temperature at which the sodium carbonate crystallization occurredin the crystallizer 48 and the relative concentrations of sodiumcarbonate and sodium chloride in the mother liquor. The temperature mustbe higher than a temperature at which sodium carbonate will crystallize,if substantially pure sodium chloride'is to be recovered, andtemperatures as high as 220F may be used. At these temperatures, thetemperature to which the hot aqueous solution is cooled to crystallizehydrated sodium carbonate must be quite low, typically about 32 to 50F.Temperatures in the range of about to 140F have proved to be thedesirable range. In the typical procedure outlined above wherein the hotaqueous solution was cooled to about 60F to achieve crystallization ofhydrated sodium carbonate, the mother liquor may be heated with boilingto about F to achieve crystallization of sodium chloridel The sodiumchloride which is crystallized is removed by line 76 in substantiallypure form. Preferably, the quantity of sodium chloride removed in thismanner is substantially the same as the quantity of sodium chloridepresent in the hot aqueous solution which is subsequently cooled tocrystallize hydrated sodium carbonate, less any sodium chloride presentin the recycle stream in line 78.

The sodium chloride removed by line 76 may be put to a variety of uses.Typically, the sodium chloride is utilized to regenerate bleach plantchemicals. For example, the sodium chloride may be used to generatesodium hydroxide by electrolysis of an aqueous solution thereof, thesodium hydroxide being used in the bleach plant in line 26.Alternatively, the sodium chloride may be used to generate chlorinedioxide and chlorine by reaction with sodium chlorate and sulphuricacid, the

chlorine dioxide and chlorine being fed to the bleach plant by line 24.Further, the sodium chloride may be electrolyzed as an aqueous solutionto sodium chlorate for use in a chlorine dioxide producing reactioninvolving reduction of the sodium chlorate in an acid medium.

The evaporation of the mother liquor from the separation of sodiumcarbonate and the separation of the sodium chloride crystallizedtherefrom is illustrated as occurring in a single vessel 72. Inpractice, usually two separate vessels are used, the slurry resultingfrom the evaporation in one vessel being passed to a second vessel forseparation of the solid sodium chloride.

The mother liquor resulting from the sodium chloride separation usuallywill still contain quantities of sodium chloride and sodium carbonateand is recycled by line 78 to the smelt dissolver 42 along with theaqueous material in line 44, or, alternatively, is recycled to hotaqueous solution in line 46.

The mother liquor in line 78 may contain small quantities of sodiumhydroxide, which would not be removed from the system by the illustratedprocedure. The sodium hydroxide may arise. from weak wash water from thewashing of the calcium carbonate mud where such water is used to provideat least part of the aqueous material fed by line 44 to the smeltdissolver 42. The sodium hydroxide also may arise from small quantitiesof sodium oxide values in the smelt.

The mother liquor in line 78 may be carbonated using carbon dioxide, orin any other convenient manner, to convert such sodium hydroxide valuesto sodium carbonate, prior to passageof the mother liquor in line 78 tothe hot aqueous solution in line 46. In this way, any build up of sodiumhydroxide values in the recycled mother liquor in line 78 is avoided.

If desired, following the separation of the sodium chloride therefrom,the resulting mother liquor may be cooled again to precipitate furtherquantities of hydrated sodium carbonate.

It will be apparent, therefore, that in the process of the embodiment ofFIG. 1, the sodium carbonate and sodium chloride in the smelt areseparated one from another into substantially pure product streams whichcontain quantities of the hydrated sodium carbonate and sodium chloridewhich are substantially equal to the quantities of these materialspresent in the smelt.

Turning now to consideration of FIG. 2, there is illustrated theapplication of the present invention to a Kraft mill operation. A smelt,consisting predominantly of sodium sulfide, sodium carbonate and sodiumchloride, and also containing minor quantities of sodium sulfate andother sodium and sulfur-compounds due to inefficiences of the furnace,is passed by line 110 to a smelt dissolver 112.

As mentioned previously, in the Kraft mill process sodium hydroxide andsodium sulfide are the active pulping chemicals and sodium sulfate,another source of soda and sulfur values, is added to the black liquorto provide make up for soda and sulfur losses.

The smelt is dissolved in the smelt dissolver 112 in an aqueous materialfed by line 114, which may be water or, in part, weak wash water, toform a hot aqueous solution, commonly known as green liquor, in line116. The green liquor in line 116 may also contain recycled liquid, aswill become more apparent hereinafter.

The hot green liquor, typically having a temperature of about 200F, iscooled in a precooler 118, with evaporation of some water therefrom, ifdesired. The water evaporated in the precooler is removed by line 120.Typically, the green liquor is cooled to about lOOF in the precooler118.

The precooled green liquor is passed by line 122 to an evaporativecooler 124 wherein the precooled green liquor is subjected toevaporative cooling. Water is evaporated from the precooled greenliquor, to cause cooling of the liquor and thereby bring aboutcrystallization of a mixture of sodium carbonate, sodium sulfide andsodium sulfate, together with minor quantities of other precipitatedsodium and sulfur salts which may be present while avoidingcrystallization of sodium chloride. The water evaporated is removed byline 126. Preferably, the evaporative cooling is carried out until themother liquor is substantially saturated with respect to sodiumchloride.

Due to the rapid increase in solubility of sodium sulfide withincreasing temperature, cooling of the green liquor to a temperature ofless than about 55F, preferably about 50F, is necessary to ensurecrystallization of sodium carbonate, sodium sulfide and sodium sulfate.At these temperatures the sodium carbonate is crystallized predominantlyas the decahydrate and sodium sulfide is crystallized predominantly asthe nonahydrate, thereby removing further water from the solution.

In the illustrated embodiment, the hot green liquor is subjected to atwo-stage evaporative cooling to achieve crystallization of a mixture ofsodium carbonate, sodium sulfide and sodium sulfate in the second stage.If desired, the cooling of the green liquor to achieve the desiredprecipitation may be carried out in a single step, and may be carriedout in any convenient manner other than evaporative cooling, if desired.Further, the cooling achieved by evaporation may be enhanced by suitableadditional cooling.

It may be desired, depending on the initial concentration of the hotgreen liquor, to crystallize only sodium carbonate and sodium sulfate,leaving the sodium sulfide in the aqueous phase along with the sodiumchloride. This particular embodiment will be discussed in more detailbelow.

The slurry resulting from the evaporative cooling is diluted withrecycle mother liquor in line 128 and the diluted slurry passes by line130 to a separator 132 of any convenient construction. In the separator132, the mixture of sodium carbonate, sodium sulfide and sodium sulfateis separated from the mother liquor and is passed by line 134 to arecausticizer wherein the sodium carbonate is converted to sodiumhydroxide, the resulting white liquor being subsequently used as pulpingliquor. The sodium sulfate passes to the furnace in the subsequentlyformed black liquor.

The quantity of sodium sulfide removed by line 134 is substantially thesame as that contained in the smelt in line 110.

The quantity of sodium carbonate and sodium sulfate removed in line 134may be less than the quantity present in the smelt, and, in the case ofsodium sulfate, formed in the green liquor and, hence, a furtherseparation of sodium carbonate and sodium sulfate may be necessary. Inthe illustrated embodiment, this is carried out by passing the motherliquor from the separator 132 by lines 136 and 138 to an evaporator 140,with a portion of the mother liquor being recycled by line 128 asdiluent for the slurry formed in the evaporative cooler 124.

The mother liquor is heated and evaporated in the evaporator 140 tocause the crystallization of sodium carbonate together with the doublesalt of sodium carbonate and sodium sulfate known as burkeite watervapor is removed by line 141. Typically, the mother liquor may beevaporated at a temperature of about 220F, while avoiding substantiallythe precipitation of sodium chloride, the evaporation being continueduntil the solution is substantially saturated with sodium chloride.

The slurry of burkeite and aqueous solution is passed by line 142 to athickener 144 of any convenient construction wherein the sodiumcarbonate and burkeite is separated from the mother liquor and removedby line 146. The sodium carbonate and burkeite separated in this wax maybe passed to the recausticizer along with the solids in line 134.Alternatively, a separator may be employed instead of the thickener 144.The thickener underflow is forwarded to the smelt dissolver 112, to theprecooler 118 or to any other convenient location.

The mother liquor from the burkeite separation is passed by line 148 toan evaporator 150 wherein water is evaporated to deposit substantiallypure sodium chloride, which, after separation from the resulting motherliquor, is removed by line 152. The quantity of sodium chloride removedby line 152 preferably is substantially that contained in the smelt inline 110. The sodium chloride removed in this way may be utilized in ayconvenient manner, typically to form bleaching chemicals.

The evaporation in evaporator 150 results in water vapor which isremoved by line 154 and may be carried out at any convenienttemperature. The temperature chosen depends on the initial concentrationof the dissolved components of the mother liquor and the quantity ofsodium chloriderequired to'be removed. The evaporation of the solutionin the evaporator 150 to Y precipitate sodium "chloride generally iscarried out until the solution is substantially saturated with respectto sodium carbonate and/or sodium sulfide and/or sodium sulfate, andtypically is carried out at a tempera- -ture of about 120F.

The mother liquor resulting from the sodium chloride crystallizationcontains residual quantities of sodium carbonate, sodium sulfide, sodiumsulfate and sodium chloride and is recycled by line 156 to carbonator158.

' The mother liquor is contacted with carbon dioxide fed by line 160, inorder to convert any sodium hydroxide present in the mother liquor inline 156 to sodium carbonate, so that a build up of sodium hydroxideupon repeated recycle is prevented. As mentioned above in connectionwith the embodiment of FIG. 1, such sodiumhydroxide may arise from weakwash water and- /or from materials in the smelt.

The carbonated mother liquor is forwarded by line 162 to the hotgreenliquor in line l-l6. This maybe achieved by feeding the mother liquor inline 162 along with the-aqueous material in line 114 to dissolve thesmelt. Alternatively, the mother liquor may be added to the hotgreenliquor'in line 116 after formation thereof. I

Byrecycling the mother liquor from the sodium chloride crystallizationin this way, residual quantities of material, which, in a continuouslyoperating procedure,

- ther quantities of sodium carbonate and/or sodium sulfide and/orsodium sulfate, which may be forwarded to the recausticizer along withthe material in line 134.

In the embodiment discussed above, wherein the precipitation of sodiumsulfide is avoided when the green liquor is cooled, followingevaporation. to precipitate sodium chloride, the mother liquor resultingafter separationof the sodium chlorideconsists mainly of sodium sulfidesolution, which may beevaporated to crystallize therefrom the remainingquantities of sodium carbonate, sodium sulfate, and sodium chloride. Thesodium sulfidesolution then may be added to the sodium hydroxidesolution resulting fromthe recausticization to provide white liquor forthe recycle as pulping liquor.

Alternatively, the sodium solution may be recycled as the pulpingliquor, while part of the sodium hydroxide solution resulting from therecausticization is for- .Warded to the bleach plant for use therein ascaustic ex- ,traction liquor, or in oxygen-bleaching processes. Asanother alternative, the sodium sulfide solution may be used, inimpregnating wood chips. The crystallized solids may be recycled to thehot green liquor.

EXAMPLES Example 1 A mass balance for the embodiment of FIG. 1 producing500. tons/day of pulp was calculated from vknown solubility data .forthe system Na CO -Na- C1l-l O. 25,209 lbs/hr. of a smelt having aconstitution of 87.5% Na CO and 12.5% NaCl is dissolved in 66,774lbs/hr. of water to provide 91,667 lbs/hr. of green liquor having atemperature of about 200F.

The green liquor is mixed with 43,372 lbs/hr. of recycle mother liquorcontaining sodium carbonate in a concentration of 12.4% and sodiumchloride in a concentration of 18.4% and having a temperature of F, toprovide a combined liquid stream which is passed to an evaporativeprecooler wherein 6,034 lbs of water are removed while the temperatureis cooled to about 120F, resulting in 129,005 lbs/hr. ofprecooledliquor.

The precooled liquor then is evaporatively cooled to 59F resulting incrystallization of sodium carbonate decahydrate. 11,368 lbs/hr. ofstream are evaporated from the precooled liquor in this cooling step,and 52,085 lbs/hr. of recycle mother liquor, containing 9.24% sodiumcarbonate and 19.10% sodium chloride, is recycled to the evaporativecooler to dilute the slurry of sodium carbonate decahydrate formedtherein.

169,722 lbs/hr. of slurry is forwarded to a filter wherein 59,493lbs/hr. of sodium carbonate decahydrate are recovered, leaving 1 10,229lbs/hr. of filtrate, which is split into the mother liquor recyclestream to dilute the slurry in the evaporative cooler and into 58,144lbs/hr. feed stream to an evaporator. In the evaporator, the feedstream, containing 9.25% sodium carbonate and 19.10% sodium chloride, isheated under a reduced pressure to evaporate 1 1,929 lbs/hr. of waterfrom the solution at 120F, resulting in crystallization of sodiumchloride from the solution.

46,215 lbs/hr. of slurry from the evaporator is passed to a filterwhereon 3,159 lbs/hr. of sodium chloride are recovered, resulting in43,372 lbs/hr. of filtrate stream containing 12.4% sodium carbonate and18.4% sodium chloride. After contact with carbon dioxide to convert anysodium hydroxide values to sodium carbonate, the filtrate stream isrecycled to the green liquor.

Example 2 A mass balance for the embodiment of FIG. 2 producing 500tons/day of pulp was calculated from experimental solubility data forthe system Na SNa CO -NaC1-I-l O. 95,750 lbs/hr of green liquorcontaining 4.2% sodium sulfide, 17.6% sodium carbonate, 3.3% sodiumchloride and 0.65% sodium sulfate and having a temperature of 200F ismixed with 28,408 lbs/hr. of recycled mother liquor from sodium chlorideremoval containing 10.0% sodium sulfide, 6.2% sodium carbonate, 12.0%sodium chloride and 0.65% sodium sulfate and 7,755 lbs/hr. of burkeitethickener underflow at 225F containing 2.713 lbs/hr. of burkeite and5,043 lbs/hr of liquor containing 7.3% sodium sulfide, 4.6% sodiumcarbonate, 16.8% sodium chloride and 0.5% sodium sulfate, and themixture of the three streams is precooled to 120F with evaporation of6.397 lbs/hr. of water.

The resulting 125,516 lbs/hr. of precooled liquor containing 5.8% sodiumsulfide, 15.6% sodium carbonate, 5.9% sodium chloride and 2.2% sodiumsulfate then is evaporatively cooled to 50F, with evaporation of 12,705lbs/hr. of water. The resulting slurry containing a solid phaseconsisting of a mixture of sodium carbonate, sodium sulfide and sodiumsulfate is diluted with 56,695 lbs/hr. of recycled filtrate from thesolid phase separation containing 6.0% sodium sulfide, 5.3%

sodium carbonate, 13.8% sodium chloride and 4.1% sodium sulfate.

The 169,506 lbs/hr of slurry is passed to a filter whereon 59,327lbs/hr. of solid is separated from the liquid phase. The seperatedsolids consists of 6.8% sodium sulfide, 28.4% sodium carbonate and 1.1%so dium sulfate. The 110,179 lbs/hr. of filtrate from this solid phaseseparation is divided into two streams, one of which is recycled todilute the slurry in the evaporative cooler. The other stream, is aquantity of 53,484 lbs/hr. and containing 6.0% sodium sulfide, 5.3%sodium carbonate, 13.8% sodium chloride and 4.1% so dium sulfate ispassed to an evaporator wherein 6,814 lbs/hr. of water is evaporated ata temperature of 225F, resulting in crystallization of burkeite. Thesolid phase burkeite is recovered in the form of a thickener underflowwhich contains 738 lbs/hr. of sodium carbonate and 1,974 lbs/hr. ofsodium sulfate, as solid phase and the thickener underflow is recycledto the green liquor. The mother liquor from this separation, in aquantity of 38,914 lbs/hr. and containing 7.3% sodium sulfide, 4.6%sodium carbonate, 16.8% sodium chloride and 0.5% sodium sulfate, then isevaporated at 120F to crystallize sodium chloride therefrom. 31,533lbs/hr. of slurry from the evaporator is passed to a filter whereon3,125 lbs/hr. of sodium chloride are recovered, resulting in 28,408lbs/hr. of filtrate stream containing 10.0% sodium sulfide, 6.2% sodiumcarbonate, 12.0% sodium chloride and 0.68% sodium sulfate.

After contact with carbon dioxide to convert any sodium hydroxide valuesto sodium carbonate, the filtrate stream is recycled to the greenliquor.

SUMMARY It will be seen that the process of the present inventionprovides in its preferred embodiments, a salt removal process in whichthe need for high temperatureresistant equipment is avoided, and thequantity of water requiring evaporation is decreased, and in which adead load of sodium chloride does not recycle in the white liquor. Inthese respects, the present invention represents an improvement on thewhite liquor evaporation procedures outlined in the aformentionedCanadian Pat. No. 915,361 and its equivalent US. Pat. No. 3,746,612.

What I claim is:

1. In a pulp mill process which comprises digesting cellulosic fibrousmaterial with a pulping liquor containing at least one active pulpingchemical, separating the pulped material from spent pulping liquor andforming a solid mass containing sodium carbonate from said spent pulpingliquor, the improvement which comprises dissolving at least part of saidsolid mass in an aqueous material to form a hot aqueous solutionthereof, providing sodium chloride in said hot aqueous solution of saidsolid mass, cooling said hot aqueous solution to crystallize hydratedsodium carbonate therefrom substantially to the point of saturation ofthe cooled solution with sodium chloride, whereby said crystallizedhydrated sodium carbonate is substantially free from contamination bysodium chloride, separating said crystallized hydrated sodium carbonatefrom the resulting aqueous solution substantially saturated with sodiumchloride, evaporating said sodium chloride solution to deposit sodiumchloride therefrom, and separating said deposited sodium chloride fromthe resulting mother liquor.

2. The process of claim 1 including recycling said resulting motherliquor to said hot aqueous solution of said solid mass.

3. The process of claim 1 wherein said separated sodium carbonate isformed into an active pulping chemical and an aqueous solution thereofis recycled to provide at least part of said pulping liquor.

4. The process of claim 3 wherein said active pulping chemical is sodiumhydroxide and is formed from said sodium carbonate by forming saidseparated sodium carbonate into an aqueous solution thereof andcausticizing substantially the sodium carbonate values of said aqueoussolution to sodium hydroxide.

5. The process of claim 4 wherein said sodium hydroxide is the solepulping chemical in said pulping liquor.

6. The process of claim 4 wherein sodium hydroxide and sodium sulfideare the active pulping chemicals in said pulping liquor and said solidmass contains sodium sulfide, and including crystallizing sodium sulfidefrom said hot aqueous solution of said solid mass together with saidhydrated sodium carbonate.

7. The process of claim 6 wherein said solid mass additionally containssodium sulfate and including crys-' tallizing at least part of saidsodium sulfate from said hot aqueous solution of said solid masstogether with said hydrated sodium carbonate and sodium sulfide.

8. The process of claim 7 including the steps of, after said separationof said crystallized sodium carbonate, sodium sulfide and sodium sulfateand before said evaporation of said sodium chloride solution,crystallizing additional quantities of sodium carbonate and sodiumsulfate and removing said additionallycrystallized sodium carbonate andsodium sulfate from the resulting mother liquor.

9. The process of claim 1 wherein sodium hydroxide and sodium sulfideare the active pulping chemicals in said pulping liquor and said solidmass contains sodium sulfide, and including crystallizing hydratedsodium carbonate from said hot aqueous solution of said solid mass whileavoiding precipitation of sodium sulfide and sodium chloride, afterseparation of said crystallized hydrated sodium carbonate, evaporatingthe resulting sodium sulfide solution to deposit substantially all thesodium chloride and sodium carbonate values therefrom, separating thedeposited mixture of sodium chloride and sodium carbonate from theresulting concentrated sodium sulfide solution, recovering substantiallypure sodium chloride from said deposited mixture and recycling theremainder of said deposited mixture to said hot aqueous solution.

10. The process of claim 1 wherein said hot aqueous solution is cooledwhile water is evaporated from the solution.

1 1. The process of claim 5 wherein said hot aqueous solution is cooledfrom a temperature of about 180 to about 210F to a temperature belowabout 68F while water is evaporated from said solution and sodiumcarbonate decahydrate is crystallized in a quantity substantiallyequivalent to the quantity of sodium carbon ate present in the solidmass.

12. The process of claim 11 wherein said hot aqueous solution is cooledinitially from a temperature of about 200F to about F and subsequentlyis cooled to a temperature of about 60F, resulting in thecrystallization of sodium carbonate decahydrate from the cooledsolution.

13. The process of claim 12 wherein said initial cooling and saidsubsequent cooling are achieved by boiling said aqueous solution under areduced pressure.

14. The process of claim 1 wherein said sodium chloride solution isevaporated to deposit substantially pure sodium chloride therefrom byboiling said solution under a reduced pressure at a temperature greaterthan the temperature to which said hot aqueous solution is cooled tocrystallize said sodium carbonate.

15. The process of claim 11 wherein said sodium chloride solution isevaporated by boiling said solution under a reduced pressure at atemperature of about 100 to 140F to deposit substantially pure sodiumchloride therefrom in a quantity substantially equivalent to thequantity of sodium chloride provided in said hot aqueous solution lessany sodium chloride recycled thereto in mother liquor from said sodiumchloride crystallization.

16. The process of claim 12 wherein said sodium chloride solution isevaporated to deposit substantially pure sodium chloride therefrom byboiling said solution under a reduced pressure at a temperature of about120F.

17. The process of claim 15 wherein said evaporation is continued untilthe mother liquor from said sodium chloride deposition is substantiallysaturated with respect to sodium carbonate.

18. The process of claim 7 wherein said hot aqueous solution is cooledfrom a temperature of about 180 to 210F to a temperature below about 55Fwhile water is evaporated from said solution, and sodium carbonatedecahydrate and sodium sulfide nonahydrate are crystallized from the hotaqueous solution in a quantity substantially equivalent to the quantityof sodium carbonate and sodium sulfide present in the solid mass.

19. The process of claim 18 wherein said hot aqueous solution is cooledinitially from a temperature of about 200F to about 100F andsubsequently is cooled to a temperature of about 50F, resulting in thecrystallization of sodium carbonate decahydrate and sodium sulfidenonahydrate from the cooled solution.

20. The process of claim 19 wherein said initial cooling and saidsubsequent cooling are achieved by boiling said aqueous solution under areduced pressure.

21. The process of claim 16 wherein said sodium chloride solution isevaporated by boiling said solution under a reduced pressure at atemperature of about 100 to 140F to deposit substantially pure sodiumchloride therefrom in a quantity substantially equivalent to thequantity of sodium chloride provided in said hot aqueous solution lessany sodium chloride recycled thereto in mother liquor from said sodiumchloride crystallization.

22. The process of claim 19 wherein said sodium chloride solution isevaporated to deposit substantially pure sodium chloride therefrom byboiling said solution under a reduced pressure at a temperature of about120F.

23. The process of claim 21 wherein said evaporation is continued untilthe mother liquor from said sodium chloride deposition is substantiallysaturated with respect to sodium carbonate.

24. The process of claim 1 wherein said solid mass additionally containssodium chloride and said latter sodium chloride constitutes the sourceof said sodium chloride provided in said hot aqueous solution of saidsolid mass.

25. The process of claim 24 wherein said hot aqueous solution is formedby contacting said solid mass with an aqueous material to dissolvesubstantially all said sodium chloride therefrom together with part ofsaid sodium carbonate therefrom, and leaving a further solid masscontaining sodium carbonate and being substantially free from sodiumchloride.

26. The process of claim 25 including utilizing the sodium carbonatecontent of said substantially sodium chloride-free solid mass to form asodium hydroxide solution for recycle as part of said pulping liquor.

27. The process of claim 2 wherein said recycling mother liquor containssodium hydroxide values and including carbonating said sodium hydroxidevalues to sodium carbonate prior to said recycle.

28. The process of claim 1 wherein said cellulosic fibrous material iswood.

29. The process of claim 28 including subjecting said pulped material toa series of bleaching and purification steps using at leastonechlorine-containing bleaching chemical in at least one of saidbleaching steps and aqueous sodium hydroxide-containing solutions insaid purification steps, discharging a sodium chloridecontaining aqueouseffluent from said series of bleaching and purification steps into saidspent pulping liquor whereby said solid mass and said hot aqueoussolution contain sodium chloride.

30. The process of claim 29 including washing said pulp after formationthereof and before passage to said series of bleaching and purificationsteps and utilizing said sodium chloride-containg aqueous effluent insaid washing.

31. The process of claim 29 wherein said series of bleaching andpurification steps involves a first stage bleaching with chlorine,chlorine dioxide or a mixture thereof, a first stage caustic extractionusing sodium hydroxide solution, a second stage bleaching with chlorinedioxide, a second stage caustic extraction with sodium hydroxidesolution and a third stage bleaching with chlorine dioxide, the quantityof sodium hydroxide used being equivalent to the quantity of chlorineused in said bleaching steps, the spent bleaching and caustic extractionbeing mixed to provide a sodium chloride containing aqueous effluent ofsubstantially neutral pH.

32. The process of claim 31 including separating said pulped materialand spent pulping liquor while washing said pulp after formation thereofand before passage to said series of bleaching and purificationoperations, washing said pulp after each said bleaching and causticextraction, the wash water utilized in said latter washing passingcountercurrently to the pulp through said series of bleaching andpurification steps, mixing the spent wash water from the latter washingwith the spent bleaching and caustic extraction chemicals to provide thesodium chloride-containing aqueous effluent and utilizing said lattereffluent in said washing of said pulp prior to passage to said series ofbleaching and purification steps, whereby said latter effluent isintroduced into said spent pulping liquor.

33. The process of claim 31 wherein said bleaching and causticextraction steps are carried out by per colating an aqueous solution ofthe treating chemical through a mass of the pulp fibers whilemaintaining the fibers relatively stationary with respect to each otherexcept for movement caused by passage of said aqueous solution throughsaid mass of the pulp fibers.

=I l l= l

1. IN A PULP MILL PROCESS WITH COMPRISES DISGESTING CELLULOSIC FIBROUSMATERIAL WITH A PULPING LIQUOR CONTAINING AT LEAST ONE ACTIVE PULPINGCHEMICAL, SEPERATING THE PULPED MATERIAL FROM SPENT PULPING LIQUOR ANDFORMING A SOLID MASS CONTAINING SODIUM CARBONATE FRON SAID SPENT PULPINGLIQUOR, THE IMPROVEMENT WHICH COMPRISES DISSOLVING AT LEAST PART OF SAIDSOLID MASS IN AN AQUEOUS MATERIAL TO FORM A HOT AQUEOUS SOLUTIONTHEREOF, PROVIDING SODIUM CHLORIDE IN SAID HOT AQUEOUS SOLUTION SAIDSOLID MASS, COOLING SAID HOT AQUEOUS SOLUTION TO CRYSTALLIZE HYDRATEDSODIUM CORBONATE THEREFROM SUBSTANTIALLY TO THE POINT OF SATURATION OFTE COOLED SOLUTION WITH SODIUM CHOLORIDE, WHEREBY SAID CRYSTALLIZEDHYDRATED SODIUM CARBONATE IS SUBSTANTIALLY FREE FROM CONTAMINAION BYSODIUM CHLORIDE, SEPERATING SAID CRYSTALLIZED HYDRATED SODIUM CARBONATEFROM THE RESULTING AQUEOUS SOLUTION SUBSTANTIALLY SATURATED WITH SODIUMCHLORIDE, EVAPORATING SAID SODIUM CHLORIDE SOLUTION TO DEPOSIT SODIUMCHLORIDE THEREFROM, AND SEPARATING SAID DEPOSITED SODIUM CHLORIDE FROMTHE RESULTING MOTHER LIQUOR.
 2. The process of claim 1 includingrecycling said resulting mother liquor to said hot aqueous solution ofsaid solid mass.
 3. The process of claim 1 wherein said separated sodiumcarbonate is formed into an active pulping chemical and an aqueoussolution thereof is recycled to provide at least part of said pulpingliquor.
 4. The process of claim 3 wherein said active pulping chemicalis sodium hydroxide and is formed from said sodium carbonate by formingsaid separated sodium carbonate into an aqueous solution thereof andcausticizing substantially the sodium carbonate values of said aqueoussolution to sodium hydroxide.
 5. The process of claim 4 wherein saidsodium hydroxide is the sole pulping chemical in said pulping liquor. 6.The process of claim 4 wherein sodium hydroxide and sodium sulfide arethe active pulping chemicals in said pulping liquor and said solid masscontains sodium sulfide, and including crystallizing sodium sulfide fromsaid hot aqueous solution of said solid mass together with said hydratedsodium carbonate.
 7. The process of claim 6 wherein said solid massadditionally contains sodium sulfate and including crystallizing atleast part of said sodium sulfate from said hot aqueous solution of saidsolid mass together with said hydrated sodium carbonate and sodiumsulfide.
 8. The process of claim 7 including the steps of, after saidseparation of said crystallized sodium carbonate, sodium sulfide andsodium sulfate and before said evaporation of said sodium chloridesolution, crystallizing additional quantities of sodium carbonate andsodium sulfate and removing said additionally-crystallized sodiumcarbonate and sodium sulfate from the resulting mother liquor.
 9. Theprocess of claim 1 wherein sodium hydroxide and sodium sulfide are theactive pulping chemicals in said pulping liquor and said solid masscontains sodium sulfide, and including crystallizing hydrated sodiumcarbonate from said hot aqueous solution of said solid mass whileavoiding precipitation of sodium sulfide and sodium chloride, afterseparation of said crystallized hydrated sodium carbonate, evaporatingthe resulting sodium sulfide solution to deposit substantially all thesodium chloride and sodium carbonate values therefrom, separating thedeposited mixture of sodium chloride and sodium carbonate from theresulting concentrated sodium sulfide solution, recovering substantiallypure sodium chloride from said deposited mixture and recycling theremainder of said deposited mixture to said hot aqueous solution. 10.The process of claim 1 wherein said hot aqueous sOlution is cooled whilewater is evaporated from the solution.
 11. The process of claim 5wherein said hot aqueous solution is cooled from a temperature of about180* to about 210*F to a temperature below about 68*F while water isevaporated from said solution and sodium carbonate decahydrate iscrystallized in a quantity substantially equivalent to the quantity ofsodium carbonate present in the solid mass.
 12. The process of claim 11wherein said hot aqueous solution is cooled initially from a temperatureof about 200*F to about 120*F and subsequently is cooled to atemperature of about 60*F, resulting in the crystallization of sodiumcarbonate decahydrate from the cooled solution.
 13. The process of claim12 wherein said initial cooling and said subsequent cooling are achievedby boiling said aqueous solution under a reduced pressure.
 14. Theprocess of claim 1 wherein said sodium chloride solution is evaporatedto deposit substantially pure sodium chloride therefrom by boiling saidsolution under a reduced pressure at a temperature greater than thetemperature to which said hot aqueous solution is cooled to crystallizesaid sodium carbonate.
 15. The process of claim 11 wherein said sodiumchloride solution is evaporated by boiling said solution under a reducedpressure at a temperature of about 100* to 140*F to depositsubstantially pure sodium chloride therefrom in a quantity substantiallyequivalent to the quantity of sodium chloride provided in said hotaqueous solution less any sodium chloride recycled thereto in motherliquor from said sodium chloride crystallization.
 16. The process ofclaim 12 wherein said sodium chloride solution is evaporated to depositsubstantially pure sodium chloride therefrom by boiling said solutionunder a reduced pressure at a temperature of about 120*F.
 17. Theprocess of claim 15 wherein said evaporation is continued until themother liquor from said sodium chloride deposition is substantiallysaturated with respect to sodium carbonate.
 18. The process of claim 7wherein said hot aqueous solution is cooled from a temperature of about180* to 210*F to a temperature below about 55*F while water isevaporated from said solution, and sodium carbonate decahydrate andsodium sulfide nonahydrate are crystallized from the hot aqueoussolution in a quantity substantially equivalent to the quantity ofsodium carbonate and sodium sulfide present in the solid mass.
 19. Theprocess of claim 18 wherein said hot aqueous solution is cooledinitially from a temperature of about 200*F to about 100*F andsubsequently is cooled to a temperature of about 50*F, resulting in thecrystallization of sodium carbonate decahydrate and sodium sulfidenonahydrate from the cooled solution.
 20. The process of claim 19wherein said initial cooling and said subsequent cooling are achieved byboiling said aqueous solution under a reduced pressure.
 21. The processof claim 16 wherein said sodium chloride solution is evaporated byboiling said solution under a reduced pressure at a temperature of about100* to 140*F to deposit substantially pure sodium chloride therefrom ina quantity substantially equivalent to the quantity of sodium chlorideprovided in said hot aqueous solution less any sodium chloride recycledthereto in mother liquor from said sodium chloride crystallization. 22.The process of claim 19 wherein said sodium chloride solution isevaporated to deposit substantially pure sodium chloride therefrom byboiling said solution under a reduced pressure at a temperature of about120*F.
 23. The process of claim 21 wherein said evaporation is continueduntil the mother liquor from said sodium chloride deposition issubstantially saturated with respect to sodium carbonate.
 24. Theprocess of claim 1 wherein said solid mass additionally contAins sodiumchloride and said latter sodium chloride constitutes the source of saidsodium chloride provided in said hot aqueous solution of said solidmass.
 25. The process of claim 24 wherein said hot aqueous solution isformed by contacting said solid mass with an aqueous material todissolve substantially all said sodium chloride therefrom together withpart of said sodium carbonate therefrom, and leaving a further solidmass containing sodium carbonate and being substantially free fromsodium chloride.
 26. The process of claim 25 including utilizing thesodium carbonate content of said substantially sodium chloride-freesolid mass to form a sodium hydroxide solution for recycle as part ofsaid pulping liquor.
 27. The process of claim 2 wherein said recyclingmother liquor contains sodium hydroxide values and including carbonatingsaid sodium hydroxide values to sodium carbonate prior to said recycle.28. The process of claim 1 wherein said cellulosic fibrous material iswood.
 29. The process of claim 28 including subjecting said pulpedmaterial to a series of bleaching and purification steps using at leastone chlorine-containing bleaching chemical in at least one of saidbleaching steps and aqueous sodium hydroxide-containing solutions insaid purification steps, discharging a sodium chloride-containingaqueous effluent from said series of bleaching and purification stepsinto said spent pulping liquor whereby said solid mass and said hotaqueous solution contain sodium chloride.
 30. The process of claim 29including washing said pulp after formation thereof and before passageto said series of bleaching and purification steps and utilizing saidsodium chloride-containg aqueous effluent in said washing.
 31. Theprocess of claim 29 wherein said series of bleaching and purificationsteps involves a first stage bleaching with chlorine, chlorine dioxideor a mixture thereof, a first stage caustic extraction using sodiumhydroxide solution, a second stage bleaching with chlorine dioxide, asecond stage caustic extraction with sodium hydroxide solution and athird stage bleaching with chlorine dioxide, the quantity of sodiumhydroxide used being equivalent to the quantity of chlorine used in saidbleaching steps, the spent bleaching and caustic extraction being mixedto provide a sodium chloride containing aqueous effluent ofsubstantially neutral pH.
 32. The process of claim 31 includingseparating said pulped material and spent pulping liquor while washingsaid pulp after formation thereof and before passage to said series ofbleaching and purification operations, washing said pulp after each saidbleaching and caustic extraction, the wash water utilized in said latterwashing passing countercurrently to the pulp through said series ofbleaching and purification steps, mixing the spent wash water from thelatter washing with the spent bleaching and caustic extraction chemicalsto provide the sodium chloride-containing aqueous effluent and utilizingsaid latter effluent in said washing of said pulp prior to passage tosaid series of bleaching and purification steps, whereby said lattereffluent is introduced into said spent pulping liquor.
 33. The processof claim 31 wherein said bleaching and caustic extraction steps arecarried out by per colating an aqueous solution of the treating chemicalthrough a mass of the pulp fibers while maintaining the fibersrelatively stationary with respect to each other except for movementcaused by passage of said aqueous solution through said mass of the pulpfibers.